In the mobile communication system, user equipment (UE) needs to execute an initial access procedure before exchanging information with a base station, so as to realize timing synchronization and finish searching for a cell (acquiring a cell ID) or searching for a cell group (acquiring a cell group ID). The timing synchronization includes symbol synchronization and frame synchronization. Generally, the UE realizes symbol synchronization by detecting a primary-synchronization (P-SCH) signal sent by the base station, and realizes frame synchronization by detecting a secondary-synchronization (S-SCH) signal sent by the base station. The S-SCH signal carries information of a cell ID or a cell group ID, so as to enable the UE to finish searching for a cell or a cell group while realizing the frame synchronization.
In the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) standard, each radio frame (10 ms) has two S-SCH signals which are respectively located in orthogonal frequency division multiplexing (OFDM) symbols of the 0th sub-frame and the 5th sub-frame, and the transmission interval between them is 5 ms. Each of the S-SCH signals includes two secondary synchronization code (SSC) sequences, and each SSC sequence is selected from a sequence set formed by M sequences with a length of 31 respectively. The sequence set is referred to as an SSC sequence set, and each SSC sequence is an element of the SSC sequence set. Thus, the two SSC sequences included in each S-SCH signal of each radio frame are generally represented as (Sa, Sb), in which a and b respectively represent the indices of the two SSC sequences in the above sequence set.
The values of index a and b are referred to as short codes, so that the S-SCH including Sa and Sb may be identified as a codeword [a, b] formed by the indices a and b, which is called an S-SCH codeword. S-SCH codewords formed by different values of a and b represent different S-SCH signals, and each of the two S-SCH signals correspond to one of 170 cell IDs or cell group IDs. In this manner, 340 different S-SCH signals are required, and correspondingly, 340 S-SCH codewords exist.
In order to detect an S-SCH signal, the UE correlates the received S-SCH signal with all the SSC sequences in the SSC sequence set, and identifies two SSC sequences corresponding to the relevant peak values (i.e. short codes in the S-SCH codeword).
In order to further reduce the synchronization time of the UE, two S-SCH signals in the same frame are set to include two same SSC sequences, but the two SSC sequences have different arranging order in each S-SCH signal. That is to say, the two SSC sequences included in the S-SCH signal of the sub-frame 0 are sequentially (Sa, Sb), and the two SSC sequences included in the S-SCH signal of the sub-frame 5 are sequentially (Sb, Sa). Thus, the UE can realize the frame synchronization and acquire the cell group ID only through detecting the S-SCH signal in one sub-frame.
Based on the above method, 170 S-SCH codewords need to be set as each corresponding to each cell ID or cell group ID. It is assumed that, among the 170 S-SCH codewords, the S-SCH codeword (corresponding to the S-SCH signal in the sub-frame 0) corresponding to the cell IDi of the ith cell is represented as ci=[si0,si1], i=0˜169, in which Si0 indicates a first short code (i.e. the short code a), Si1 indicates a second short code (i.e. the short code b); and then the first short code and the second short code are reversed, so as to obtain the other 170 S-SCH codewords (corresponding to the S-SCH signals in the sub-frame 5) ci=[si1,si0], i=0˜169, which have a reversed short code sequence.
At the boundaries of a plurality of cells, the UE may detect SSC sequences from many different cells, and the detected SSC sequences from each cell have similar correlation value, so that the S-SCH signals detected by the UE may be formed by SSC sequences respectively from two different cells. For example, if the S-SCH codeword ci of the cell i is [si0,si1], and the S-SCH codeword cj of the cell j is └sj0,sj1┘, the UE may incorrectly detect the S-SCH signals such as └si0,sj1┘ or └sj0,si1┘ including the SSC sequences respectively from two different cells at the boundary of the two cells. Apparently, such S-SCH signals are invalid, and the UE cannot obtain the frame synchronization or acquire the cell group ID. Thus, in order to reduce the occurrence of the above circumstance and enhance the reliability of the frame synchronization, all the set S-SCH codewords need to fulfill si0<si1 (or si0>si1), that is, the magnitude relation between the first short code and the second short code in each S-SCH codeword needs to be the same, and the maximum short code distance |si0-si1| between two short codes needs to be as small as possible. If all the S-SCH codewords satisfy si0<si1 (or si0>si1), the UE may realize the frame synchronization even if the UE does not acquire the cell group ID.
An encoding method for an S-SCH codeword is proposed in the related art, which may be used to obtain all the S-SCH codewords and a cell ID or a cell group ID corresponding to each S-SCH codeword.
Particularly, the encoding method may be represented as:
                                                        s              0                        ⁡                          (              ID              )                                =                      mod            ⁡                          (                              ID                ,                31                            )                                      ⁢                                  ⁢                              s            1                    ⁡                      (            ID            )                          =                  mod          ⁡                      (                                                                                s                    0                                    ⁡                                      (                    ID                    )                                                  +                                  ⌊                                      ID                    31                                    ⌋                                +                1                            ,              31                        )                                              Equation        ⁢                                  ⁢        1            
The variable ID indicates a cell ID or cell group ID, s0 (ID) indicates a first short code corresponding to the cell ID or cell group ID, s1 (ID) indicates a second short code corresponding to the cell ID or cell group ID, 31 indicates the number of elements in the SSC sequence set, and
  ⌊      ID    31    ⌋indicates the maximum integer smaller than the value of a quotient of dividing the cell ID or cell group ID by the SSC sequence length.
During the process of realizing the above encoding solution, the inventor(s) found that the related art has the following problem: the S-SCH codewords obtained through the above encoding method may not all satisfy the conditions that, the first short code in each generated S-SCH codeword is larger than the second short code, or the first short code in each generated S-SCH codeword is smaller than the second short code, and the short code distance shall be as small as possible, and as a result, it may not ensure the reliability of the frame synchronization.